The first description of a process for the production of basic aluminum nitrate that might be suitable for incorporation into a nitric acid-based process for the recovery of aluminum oxide from kaolin was by Thor Mejdell in U.S. Pat. No. 1,413,754 and Canadian Pat. No. 224,026. Mejdell's process was improved upon by R. B. Derr and H. P. Stere, U.S. Pat. No. 2,127,504, Aug. 23, 1938. Judging from the paucity of information on this subject since that time, little or nothing has been done either to commercialize or to improve upon the cited developments until the efforts of the instant inventors and their studies which culminated in the inventions to be described herein. From the results of extensive studies we now believe that the cited art has not been commercialized because:
1. The basic aluminum nitrate crystals produced were extremely fine, substantially needles of about 1 micron in diameter by 10 microns long, which are extremely difficult to separate from the mother liquors, and PA1 2. Sustantial quantities of water had to be introduced which diluted the acid and required more heat for evaporation. PA1 (1) ANN (Al.sub.2 O.sub.3.6HNO.sub.3.15H.sub.2 O or Al(NO.sub.3).sub.3.9H.sub.2 O), and PA1 (2) The desired crystal product BAN or Wiseite PA1 (a) BAN (Al.sub.2 O.sub.3.2HNO.sub.3.5H.sub.2 O or Al(OH).sub.2 NO.sub.3.2H.sub.2 O), hereinafter referred to as the BAN-ANN Join, if BAN is desired, or PA1 (b) Wiseite (Al.sub.2 O.sub.3.2HNO.sub.3.4H.sub.2 O or Al(OH).sub.2 NO.sub.3.5H.sub.2 O), hereinafter referred to as the Wiseite-ANN Join, if Wiseite is desired.
Other people have made basic aluminum nitrates for various uses as, for instance, Wolkober, British Pat. No. 1,184,730, published Mar. 18, 1970, who evaporated some nitric acid and water from crystalline aluminum nitrate nonahydrate and then refluxed the residue for 8 hours to obtain a material that was a good additive to polyvinylchloride; Max Buchner, U.S. Pat. No. 1,792,410, who evaporated aluminum nitrate-containing liquors under vacuum to produce an alumina material and thereby of necessity produced a basic aluminum nitrate material; but these and the like failed to address themselves to the problems of (1) using the preparation step to obtain a purification of the crystals as cited by Mejdell and (2) providing the vapors of nitric acid and water at a temperature and pressure satisfactorily for recovering the heat of condensation for reuse at other places in the process. Thus, Wolkober refluxes the material for 8 hours and Buchner does his evaporation under a vacuum.
Mejdell taught the establishment of a pool of aluminum nitrate-containing liquor in a container at atmospheric pressure, or in an autoclave for operation under increased pressure, maintaining the temperature of the pool at between about 140.degree. and 145.degree. C. by the application of heat while simultaneously removing vapors from above the pool and replenishing the liquid in the pool with aluminum nitrate solution and/or steam or water as needed to maintain the temperature of the pool between about 140.degree. and 145.degree. C. An appreciable amount of the alumina precipitated as a crystalline basic aluminum nitrate poor in water. Since the precipitate was crystalline it could be filtered out of the material relatively easily. Further, the concentration of iron in these crystals was substantially less than that in the mothor liquor (we have found the same to be true for potash, phosphorus, and a number of other impurity elements) so that the step can be used as a part of the purification operation required to produce the chemically pure alumina demanded by present-day alumina reduction cells. Mejdell's vapors contained no nitrous gases and, since they were present at one atmosphere pressure or higher, could be condensed at temperatures upward of 100.degree. C. in suitable means whereby the heat of condensation could be recovered and recycled to the process. The crystals were especially suitable as feed to the decomposers where alumina was produced by thermally decomposing the basic aluminum nitrate into alumina, nitrous gases and water vapor, because the crystals did not melt or soften or otherwise become sticky and ball up as was the case when efforts were made to decompose aluminum nitrate nonahydrate in the same types of equipment.
Derr and Stere improved upon Mejdell by introducing a step of first evaporating from the molten aluminum nitrate nonohydrate about 30% of the contained nitric acid and water before feeding the residual strong liquor into the steam-heated pool of Mejdell, and by limiting the total evaporation of nitric acid to about 50% of that contained in the original molten aluminum nitrate nonahydrate in one pass. Basic aluminum nitrate crystals were separated for subsequent calcining and the unprecipitated alumina was recycled. Since the first portion of nitric acid was recovered as about 48% by weight nitric acid solution and the second portion can be about a 35% by weight solution, the overall recovered acid concentration was substantially higher than the 16-20% cited by Mejdell. This represents a very substantial savings in dilution over Mejdell and therefore a very substantial potential savings in energy requirements by Mejdell. Indeed, we have found that the use of the heat of condensation recoverable from the vapors so produced would permit a substantial savings in fuel usage for the manufacture of metallurgical grade alumina from kaolin over other processes producing comparable purity alumina without this step.
However, we have found it exceedingly difficult to realize the potential heat recovery and purification aspects of the production of basic aluminum nitrate because of the very small crystals obtained when making basic aluminum nitrate according to the cited prior art. This has been especially true in attempts to practice the process on a continuous basis such as would be needed for a large alumina-from-clay production facility. Crystals produced have been, predominantly, about 1 micron diameter by 10 microns long with some crystals in the area of, for instance, 5 micron diameter by 30 microns long and a relative few approaching a large size of 15 microns diameter by 45 microns long. We have been able to recover these crystals in laboratory batch type centrifuges lined with fine filter cloth but the capital cost and power consumption of this type equipment for the production of, for example, 10,000 tons/day of crystals is impossible to absorb. Consequently, a principal objective of the instant invention is to produce basic aluminum nitrate crystals of such larger size that they can be separated and washed easily on continuous centrifuges of, for instance, pusher or screen bowl types, as would be required for a commercial operation.